In offshore petroleum operations, platforms comprising a trussed steel framework, known as a "jacket", secured to the seafloor and a deck mounted on top of the jacket are commonly used to drill for and produce oil and gas. Typically, the deck is mated to the jacket after the jacket has been installed by lifting individual components of the deck, known as "modules", including deck sections, crew facilities, and drilling and production equipment, onto the jacket with a barge-mounted crane. After the individual modules are lifted onto the jacket, they are interconnected and commissioned.
This approach generally works quite well, however, costs can be very high due to the extensive offshore construction required. Offshore construction is very expensive for a number of reasons, including down-time caused by rough weather and the need for special offshore construction vessels. In the case of very large platforms, or platforms located in remote areas, offshore construction may require many months and millions of man-hours to complete.
An alternate approach to mating platform decks and jackets, known as the "integrated deck approach", has been introduced in recent years. With the integrated deck approach, a one-piece deck is used, with most or all components being integrated at an onshore construction yard. By using the integrated deck approach, offshore construction time is greatly reduced. This not only substantially reduces offshore construction costs, but it also makes this approach attractive for offshore areas having short construction seasons due to rough seas or the presence of sea ice.
Because an integrated deck consists of a single unit comprising most or all of the modules used for drilling and Production, it can be very heavy. Integrated decks having total weights of 40,000 tons or more have been proposed. For this reason, integrated decks are not lifted onto platform jackets with barge-mounted cranes. Instead, the integrated deck is carried to the jacket on a barge, and the barge is then ballasted to lower the integrated deck onto the jacket. Typically, the jacket will have a slot into which the barge is maneuvered. The integrated deck extends over both sides of the barge and mates with the jacket as the barge is ballasted.
Because the integrated deck is carried by a barge during the mating operation, it is subject to movement caused by the action of wind on the barge and deck, and more importantly, by the action of waves and currents on the barge. This movement can make proper alignment of the integrated deck with the jacket very difficult. Moreover, large sudden movements of the integrated deck resulting from motions of the barge can cause the deck legs to slam into the jacket, thereby damaging the legs and/or the jacket.
Although various apparatus for aligning and mating integrated decks with jackets have been used and proposed, these apparatus are generally not satisfactory for use in moderate sea states or are too complicated and expensive to be practical. The integrated deck approach is therefore currently limited to areas where higher seas are not likely during the mating operation, and as a result, the advantages to using this approach currently cannot be realized to the extent desired by the petroleum industry. For this reason, there is a need for a practical system and method which permits the alignment and mating of an integrated deck with a jacket in higher seas. The present invention is aimed at providing such a system and method.